biosphere

biosphere The biosphere is the part of our planet that is inhabited by life. It consists of three zones: the lithosphere, comprising the Earth's crust, together with accumulated soils and sediments; the hydrosphere, that part of the lithosphere covered in water, including groundwater; and the atmosphere, the gaseous envelope surrounding the Earth. Life on Earth is extremely ancient, having evolved before 3.8 billion years (Ga) ago. It has played, and still plays, a major role in the development of our planet and its climate.

The greatest biomass, and largest variety, of living organisms inhabit the low-altitude surface lithosphere and upper hydrosphere. Most organisms rely on photosynthesis, and thus on solar energy, for their existence, either directly (plants and some bacteria) or indirectly (animals and most microorganisms). Life continues to greater depths and higher altitudes away from this surface layer. However, the further away from this layer the more reduced the variety, and density, of organisms becomes—as, for example, when ascending mountains. Numerically, bacteria are the dominant form of life (total 4–6 × 10³⁰ cells); they also represent a major component of the total biomass on Earth. This is because they can grow under a wide range of conditions (for example, at −5 to 113 °C, at pH 0 to 11, in near-vacuum or at pressures 1000 times greater than atmospheric, and in distilled water or in saturated salt solution). They thus greatly extend the biosphere.

In the upper atmosphere life becomes very restricted; some, indeed, question whether this is a true habitat. Nevertheless, some viable bacterial cells and bacterial and fungal spores are present. Growth and multiplication of micro-organisms here is limited by lack of nutrients and water; and solar radiation can be lethal.

The hydrosphere contains living organisms throughout, although the greatest abundance of organisms is in the near-surface photic zone, where light penetrates. Below this, life is adapted to live in dark, high-pressure conditions and depends primarily on dead organisms from above for food. At the sea floor, which may be as much as 11 km below sea level, life is sparse and is dominated by invertebrates that feed on falling particles and on the sediments, which contain large bacterial populations. The upper sediment layers can contain 10 000 million (10¹⁰) bacterial per millilitre. Below the depth of oxygen penetration life is restricted to anaerobic bacteria. Although they decrease in numbers from the sediment surface, bacteria have still been detected at 850 m below the sea floor (the deepest sample analysed) and even in the rocks beneath.

Where new crust is being formed at ocean ridges, unique ecosystems exist as oases of life at hydrothermal vents. The expulsion of hot magma results in deep circulation of sea water, which reacts with the hot crust and is chemically altered before being expelled as ‘black smokers’, at temperatures up to 350 °C, or as ‘white smokers’ at lower temperatures. The fluids are rich in hydrogen sulphide, methane, and dissolved reduced metals; for example, Fe²⁺ in the presence of oxygen in sea water provides a chemical energy source for high-temperature bacteria. This energy is used to produce new organic compounds form carbon dioxide, and thus enables bacteria to grow and divide in the absence of light. Some bacteria do not even need oxygen and thus are considered to be completely independent of photosynthesis, using geothermal rather than solar energy. The mass of bacteria around vents can be so great that they provide food, locally, for vast numbers of clams, shrimps, and tube worms.

In the lithosphere, soils and sediments contain mainly invertebrates and micro-organisms. But bacteria also live deep, to at least 3.5 km depth, in the cracks of subsurface rocks, in aquifers, in salt and mineral mines, and even in oil reservoirs. Temperature alone should not be a limiting factor until between 5 and 10 km depth. Some of these bacterial populations obtain their energy from subsurface geochemical or geothermal processes and thus, like hydrothermal bacteria, may be independent of the surface biosphere and the sunlight that drives it.

R. John Parkes

Bibliography

Postgate, J. R. (1994) The outer reaches of life. Cambridge University Press.